The present invention relates generally to gas assisted injection molding and, more particularly, to an injection molding system and method in which flowable plastic is introduced into a mold cavity at a predetermined pressure at one location, and gas is introduced into at least one mold cavity portion remote from the first mold cavity location at a pressure corresponding to the pressure of the flowable plastic, to maintain a uniform pressure on the flowable plastic in the mold cavity during dispensing and cooling thereof.
Injection molding provides an effective method of forming plastic parts having high quality finished appearances. In conventional injection molding systems, a flowable plastic is introduced into a two-piece mold cavity at a predetermined pressure. The predetermined pressure is maintained within the mold cavity during dispensing of the flowable plastic and cooling thereof to disperse the plastic uniformly through the mold cavity to facilitate a high quality part finished appearance surface.
However, in conventional injection mold systems, and particularly in a mold cavity configured to produce a thin, elongated part, the flowable plastic material is introduced at one end of the mold cavity. As the plastic is dispensed within the cavity, the packing pressure decreases as the plastic flows further away from the dispensing gate. This drop in pressure correspondingly creates sink marks and warps in the finished part. Thus, the appearance side of the part must either be painted or the part must be discarded, thereby increasing production costs.
One conventional solution to the above problem is to introduce the thermoplastic into the mold cavity at a higher packing pressure than would otherwise be required. Therefore, as the flowable plastic is dispensed within the cavity, the plastic dispensed into a mold portion remote from the gate is maintained at or near a target pressure. However, the increased gate pressure causes the pressure on the plastic dispensed in proximity to the gate to be maintained at a higher than necessary pressure, which in turn increases the stress and strain on the plastic. This increased stress and strain causes stress and strain marks to form on the part appearance surfaces at these locations, thereby detracting from part appearance and resulting in the part appearance side having to be refinished or the part to be discarded altogether.
Another conventional approach to overcoming the above problem is to introduce gas internally into the flowable plastic and mold portions remote from the gate. By internally introducing gas at these locations, the plastic packed against the mold cavity walls, and the problems associated with introducing the flowable plastic at higher than necessary gate pressures, is avoided. By introducing gas internally into the flowable plastic, the plastic is packed at appropriate pressures throughout the mold cavity. However, gas channels are also created within the part being formed. Such a method is thus not desirable in the formation of many parts, especially elongated parts having minimal thicknesses, as the gas channels tend to lessen the structural integrity of such parts.
Therefore, in view of the above, there is a need for a gas assisted injection molding system that maintains relatively uniform packing pressure distribution on the flowable plastic dispensed into the mold cavity during dispensing of the plastic into the mold cavity and during formation of the part therein to minimize sink mark formation and stress placed on plastic introduced through the gate.